1. Field of the Invention
This invention relates to piezoelectric composites, and more particularly to piezoelectric composites for high-frequency ultrasound applications and methods of manufacturing such composites.
2. Background Art
Typically, high quality medical imaging uses ultrasonic transducers or transducer arrays that posse the properties of good sensitivity and wide frequency bandwidth. Conventional transducers utilizing monolithic piezoelectric material such as, for example, lead zirconate titanate (“PZT”), typically exhibit a large acoustic impedance mismatch between the transducer and the medium under test, such as, for example, water, human tissue, and the like. To overcome this problem, piezoelectric composites that are made of individual small piezoelectric elements surrounded and isolated by a polymer matrix, such as, for example, epoxy, have been proposed. These proposed small piezoelectric elements play an increasingly important role in the development of ultrasonic transducers for medical imaging. The most commonly used structures of piezoelectric composite consist of small strips or posts of PZT that are embedded in a host matrix of polymer material. The height of the strips or posts is normally about one half wavelength at the operating frequency.
The conventional process for the fabrication of a piezoelectric composite, the “dice and fill” method, begins with a monolithic slab of piezoelectric material. Slots, or kerfs, are cut into the slab using a dicing saw. The slots are then filled in with host material such as epoxy. A two-dimensional piezoelectric composite which consists of posts and host matrix is made by cutting the piezoelectric slab in two orthogonal directions.
The volume ratio, which is the ratio of ceramic volume over the whole composite and is usually equal to the ratio of ceramic width to pitch size in the cases of conventional 1-3 and 2-2 composites, affects characteristics of the piezoelectric composite such as coupling coefficient, velocity, acoustic impedance, and the like. Thus, changing the volume ratio allows for the customization of the particular piezoelectric composite for the desired transducer application/design. Unfortunately, in the conventional “dice and fill” process, the slot/kerf size is determined by the thickness of the saw blade. It is difficult to make a specific volume ratio composite when the pitch size needs to be fixed. Normally, the volume ratio is changed by using blades of different thickness, but the volume ratio is still limited by the thickness of the blades that can be chosen, particularly when the slots/kerfs to be cut are of fine dimensions.